Manufacturing apparatus and method for producing a preform

ABSTRACT

The present invention relates to a method of forming a preform from a material having a first edge and a second edge utilizing a mandrel. The method comprises the step of extruding the material through an extruder to form an extrudate. The first edge of the extrudate has a configuration complementary to a configuration of the second edge of the extrudate. The method also includes wrapping the material around the mandrel by abutting the first edge and the second edge to form a spiral joint. 
     The invention uses a manufacturing apparatus comprising a multi-screw extruder having at least three intermeshing screws for forming the extrudate. The mandrel receives the extrudate and a pressure-applying device adjacent to the mandrel engages the extrudate and applies pressure to the spiral joint to create a smooth exterior surface of the extrudate and to define the preform.

RELATED APPLICATIONS

This application claims priority to and all advantages of U.S. Provisional Patent Application No. 61/132,281, which was filed on Jun. 17, 2008.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a manufacturing apparatus and a method of producing a preform.

2. Description of the Related Art

Manufacturing preforms, such as ceramic preforms, are known in the industry. However, the current methods of manufacturing ceramic preforms are accomplished through a manual batch mixing process and then a manual formation of the design configuration. Traditionally, preforms in the shape of a cylinder are hand formed from a material. A metal wire is disposed about the preform for strengthening the preform. More specifically, the material used for the production of preforms does not meet the strength requirements for the final preform. As such, the metal wire is required to add strength to the final preform. A preform having the metal wire as discussed above is disclosed in U.S. Pat. No. 6,530,458.

During the manual forming process, the material is generally rolled out onto a flat surface. The material has a first edge and a second edge. The first edge and the second edge are abutted to form the cylinder configuration of the preform. Several different joint concepts have been attempted including butt joints, dovetail joints and several variations of these concepts. However, these joint types are weak and tend to separate during subsequent processing of the preform. Further, these joint types are complicated and unlikely to be formed in an automation process.

Therefore, there exists a need for a joint that is strong enough not to separate during processing and that is efficient to manufacture thereby allowing for the automation of the process to decrease production time and production costs.

SUMMARY OF THE INVENTION AND ADVANTAGES

The present invention relates to a manufacturing apparatus for producing a preform. In one embodiment, the manufacturing apparatus comprises a multi-screw extruder having at least three intermeshing screws for producing an extrudate. The extrudate has a first edge and a second edge. The manufacturing apparatus also includes a mandrel for receiving the extrudate by wrapping the extrudate around the mandrel to form a spiral joint by overlapping and abutting the first edge with the second edge. The manufacturing apparatus further includes a pressure-applying device adjacent to the mandrel for engaging the extrudate and applying pressure to the spiral to level an exterior surface of the extrudate and to define the preform.

The present invention also includes a method of forming the preform from the material utilizing the mandrel. The method comprises the step of forming the first edge and the second edge of the material with the first edge having a configuration complementary to a configuration of the second edge. The method also includes the steps of wrapping the material around the mandrel and abutting the first edge and the second edge to define a spiral joint. Pressure is applied to the spiral joint to level the exterior surface of the material and to define the preform.

In another embodiment of the method, the preform comprises ceramic particles and ceramic fibers with an aspect ratio of greater than 3:1 and the ceramic fibers substantially randomly orientated in three dimensions. The method can also include the step of extruding the ceramic particles and the ceramic fibers through the extruder to form the extrudate having the first and second edges.

The spiral joint, and the manner in which the preform is manufactured, creates a consistently strong joint that does not separate during subsequent processing of the preform. The spiral joint eliminates any sharp discontinuities that may arise due to a gap in the preform. Additionally, utilization of the spiral joint lends itself to an automated production setting thereby decreasing production time and increasing efficiencies.

BRIEF DESCRIPTION OF THE DRAWINGS

Other advantages of the present invention will be readily appreciated, as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:

FIG. 1 is a perspective view of a manufacturing apparatus for producing a preform in accordance with the subject invention;

FIG. 2 is a perspective view of a preform;

FIG. 3 is a cross-sectional perspective view of an extrudate having a first edge and a second edge;

FIG. 4 is a cross-sectional side view of the extrudate with the first edge and the second edge abutting each other;

FIG. 5 is a perspective view of a wrapping device; and

FIG. 6 is a schematic side view of an oven.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the Figures, wherein like numerals indicate like or corresponding parts throughout the several views, a manufacturing apparatus 20 for producing a ceramic article or preform 22 from a material (not shown) is generally shown in FIG. 1. The preform 22 has a width and a final configuration defining an internal diameter Di as shown in FIG. 2.

The manufacturing apparatus 20 includes a multi-screw extruder 24 for processing the material and producing an extrudate 30. The material preferably includes ceramic particles and ceramic fibers with an aspect ratio of greater than 3:1 with the ceramic fibers being substantially randomly orientated in three dimensions. However, it is to be appreciated that the material can be manufactured from components other than ceramic particles and ceramic fibers without deviating from the broadest scope of the subject invention.

The multi-screw extruder 24 preferably has at least three intermeshing screws 25. The at least three intermeshing screws 25 are typically arranged in a fixed ring configuration and are typically geared to a common motor. The screws 25 also typically rotate at a common speed as is known in the industry. The at least three intermeshing screws 25 may be co-rotating or counter-rotating. The multi-screw extruder 24 typically has a modular design and comprises solid barrels and/or combination barrels. The combination barrels typically include ports for injecting components or for venting volatile gases. One skilled in the art typically selects a combination of solid barrels and combination barrels to provide desired mixing characteristics of the multi-screw extruder 24 and desired physical properties of the extrudate 30.

The multi-screw extruder 24 may also include flow blocking flights for providing separate mixing processes in the multi-screw extruder 24. The flow blocking flights may be flighted, and typically impede passing of the material between sections of the multi-screw extruder 24. It is to be appreciated that certain flow blocking flights can be removed for increasing the feeding capability of the multi-screw extruder 24.

The multi-screw extruder 24 typically has about 2 to 8 mixing zones and more typically about 4 to 6 mixing zones. The multi-screw extruder 24 also typically has an L/D ratio of about 18 to 56, and more typically of about 20 to 44. A suitable multi-screw extruder 24 is the 3+ RingExtruder commercially available from Century, Inc. of Traverse City, Mich.

The process of forming or extruding the extrudate 30 may be further defined as rotating the at least three intermeshing screws 25 at about 20 to 1,200 rpm and more typically about 100 to 400 rpm. As the at least three intermeshing screws 25 rotate, the material is conveyed, mixed, and advanced through the multi-screw extruder 24 until the material exits the multi-screw extruder 24.

The multi-screw extruder 24 typically elongates and shears the material to provide distributive and dispersive mixing to both substantially randomly orient the ceramic fibers in three dimensions and substantially homogeneously distribute dispersed ceramic fiber. The step of extruding typically includes arranging adjacent ceramic fibers in different dimensions so that adjacent ceramic fibers arranged in different dimensions are present in the extrudate 30 in an amount of greater than 85 parts by volume based on 100 parts by volume of the extrudate 30. That is, the multi-screw extruder 24 typically provides excellent elongational and low-intensity shear mixing that results in adjacent ceramic fibers oriented in different dimensions in the extrudate 30.

The multi-screw extruder 24 typically also mixes an organic binder into the material. The organic binder typically comprises a cellulose ether. The cellulose ether typically exhibits reverse thermal gelation and provides lubricity during formation of the preform 22. Without intending to be limited by theory, it is believed that the cellulose ether also typically provides surface activity, plasticity, uniform rheology, strength, and uniform distribution of air during formation of the preform 22. The cellulose ether is typically selected from the group of methyl cellulose, hydroxypropylmethylcellulose, hydroxybutylmethylcellulose, and combinations thereof. A suitable methyl cellulose is hydroxypropylmethylcellulose, commercially available under the trade name Methocel™ A4M from The Dow Chemical Company of Midland, Mich. The specific features and characteristics of the material are set forth in copending U.S. patent application Ser. No. ______, filed concurrently herewith (H&H Docket No. 65109.038).

The multi-screw extruder 24 typically also mixes a filler into the material. One skilled in the art typically selects the filler to control the density of the extrudate 30. That is, the filler is typically included in the material according to the weight percent of ceramic particles and ceramic fibers in the material. The filler typically spaces out the ceramic particles and ceramic fibers to provide the extrudate 30 and the preform 22 with desired density and to allow effective metal infiltration during any secondary processing of the preform 22, such as infiltrating the preform 22 with a metal. The filler may be any filler known in the art. The filler is typically selected to burn off during heating of the extrudate 30. The filler is typically selected from walnut shell flour, cellulose fiber, air, and combinations thereof.

A suitable filler is walnut shell flour, commercially available under from Ecoshell of Corning, Calif. The specific features and characteristics of the material are set forth in copending U.S. patent application Ser. No. ______, filed concurrently herewith (H&H Docket No. 65109.038).

The multi-screw extruder 24 typically also mixes an inorganic binder into the material. The inorganic binder is typically silica. Without intending to be limited by theory, it is believed that the inorganic binder provides the preform 22 with strength. A suitable inorganic binder is silica, commercially available under the trade name Bindzil 1440 Colloidal Silica from Wesbond Corporation of Wilmington, Del. The specific features and characteristics of the material are set forth in copending U.S. patent application Ser. No. ______, filed concurrently herewith (H&H Docket No. 65109.038).

A die 26 is disposed on the multi-screw extruder 24 for forming the material as the material exits the multi-screw extruder 24. The die 26 defines a bore 28 through the die 26 for allowing the material to pass through the die 26. The material is forced through the bore 28 of the die 26 by the multi-screw extruder 24. The material defines the extrudate 30 once the material has been processed by the multi-screw extruder 24 and has been shaped by the die 26. It is to be appreciated that the material may be formed either manually or automatically without deviating from the subject invention. It is to be further appreciated that the manufacturing apparatus 20 may forego the multi-screw extruder 24 and the material may be mixed and formed by other methods known in the art, such as by manual processing, without deviating from the broadest scope of the subject invention.

Referring to FIGS. 3 and 4, the extrudate 30 in the most preferred embodiment has a rectangular cross section with an exterior surface 32 disposed between a first edge 34 and a second edge 36. The first edge 34 and the second edge 36 of the extrudate 30 define a first configuration and a second configuration, respectfully, with the first configuration being complementary to the second configuration. In one embodiment, the first configuration and the second configuration are substantially planar and angled relative to the exterior surface 32. Typically, the first edge 34 and the second edge 36 are cut at about a 45-degree angle such that each edge 34, 36 is parallel with each other. It is to be appreciated that the edges 34, 36 may be cut at any angle without deviating from the subject invention. Additionally, it is to be appreciated that the edges 34, 36 can define configurations other than angles, such as a dovetail or a notch, without deviating from the subject invention.

Referring back to FIG. 1, the manufacturing apparatus 20 also includes a mandrel 38 having a length L disposed adjacent to the multi-screw extruder 24. The mandrel 38 has a diameter that is substantially equal to the internal diameter Di of the preform 22 that is desired. It is to be appreciated that the diameter of the mandrel 38 may vary without deviating from the subject invention. Typically, the mandrel 38 is formed of a polymeric material and is heated to an elevated temperature. More specifically, the elevated temperature is a first temperature of from about 100 to 180 degrees Fahrenheit, more typically of from about 120 to 160 degrees Fahrenheit and most typically of from about 130 to 150 degrees Fahrenheit. It is to be appreciated that the mandrel 38 can comprise other materials of which would not require heating the mandrel 38 without deviating from the subject invention. The extrudate 30 is wrapped around the mandrel 38 so that the first edge 34 and the second edge 36 of the extrudate 30 overlap and abut each other to define a spiral joint 40.

The mandrel 38 is preferably disposed on a translation device 44 such that the mandrel 38 may move relative to the multi-screw extruder 24 and the die 26 for providing an unwrapped portion of the mandrel 38 adjacent to the multi-screw extruder 24 for receiving the extrudate 30. As shown, the translation device 44 incrementally moves the mandrel 38 transverse to the extruder 24 and simultaneously provides rotation of the mandrel 38 as the extrudate 30 is wrapped around the mandrel 38. It should be appreciated that the translation device 44 may be of any suitable configuration.

A substance can be applied to the first edge 34 and the second edge 36 for increasing adhesion of the extrudate 30 prior to the step of wrapping the extrudate 30 around the mandrel 38. The substance may be of any composition, including water, to increase adhesion without deviating from the subject invention.

The manufacturing apparatus 20 further includes a pressure-applying device 42 disposed adjacent to the mandrel 38. The pressure-applying device 42 engages the extrudate 30 for applying pressure to the spiral joint 40 to level the exterior surface 32 of the extrudate 30. The pressure-applying device 42 is shown schematically and may be of any suitable configuration. A second substance can be applied to the spiral joint 40 prior to or simultaneously with applying pressure to the spiral joint 40 for assisting with the leveling of the extrudate 30. It is appreciated that the second substance can also be of any composition, including water, to increase adhesion without deviating from the subject invention.

The extrudate 30 is disposed on the mandrel 38 such that a majority of the length L of the mandrel 38 is covered by the extrudate 30. It is to be appreciated that once the majority of the length L of the mandrel 38 has been wrapped with the extrudate 30 the mandrel 38 can be removed from the manufacturing apparatus 20 and replaced by a second mandrel (not shown) for repeating the process of wrapping the mandrel 38 with the extrudate 30. Preferably, the length L of the mandrel 38 is longer than the width of several of the preforms 22 such that the extrudate 30 wrapped on the mandrel 38 can be cut into the individual preforms 22 before removing the preforms 22 from the mandrel 38. The extrudate 30 defines the preform 22 once the exterior surface 32 of the extrudate 30 has been leveled and the preform 22 has been cut to size.

Turning to FIG. 5, the manufacturing apparatus 20 further includes a wrapping device 46 for disposing a film 48 about the extrudate 30 or preform 22. As shown, the wrapping device 46 preferably disposes the film 48 about the extrudate 30 while still on the mandrel 38. The film 48 is extracted from a dispenser 50, which is schematically shown in FIG. 5. The film 48 encapsulates the extrudate 30 or preform 22 to control an evaporation rate of moisture within the extrudate 30 or preform 22. In one embodiment, the film 48 is a polymeric film, such as a polyethylene film. As with the translation device 44, the wrapping device 46 incrementally moves the mandrel 38 transverse to the extruder 24 and simultaneously provides rotation of the mandrel 38 as the film 48 is wrapped around the mandrel 38. It should be appreciated that the wrapping device 46 may be of any suitable configuration. The specific features and characteristics of the film and the method of controlling the evaporation are set forth in copending U.S. patent application Ser. No. ______, filed concurrently herewith (H&H Docket No. 65109.043).

Referring to FIG. 6, the manufacturing apparatus 20 includes a heating apparatus 52, such as an oven or a kiln. The heating apparatus 52 is shown schematically in FIG. 6 and it should be appreciated that the extrudate 30 or preform 22 may be heated by other methods, such as an open heat source. The heating apparatus 52 heats the extrudate 30 or preform 22 to a plurality of elevated temperatures as discussed in greater detail below. Typically, the heating is utilized for evaporating water, burning off any organic binder and filler from the extrudate 30 or preform 22, and setting the inorganic binder to strengthen the preform 22, which defines void space in the preform 22. As shown, the mandrel 38 with the wrapped extrudate 30 or preform 22 is disposed within the oven as a single unit. It should be appreciated that the heating of the extrudate 30 or preform 22 can be accomplished through a variety of different methods.

The invention further provides an associated method for producing the preform 22 from the material. It is to be appreciated that the method herein described can be accomplished utilizing the manufacturing apparatus 20 described above or by other methods known in the art, such as by manual processing, without deviating from the broadest scope of subject invention. The method includes the step of forming the first edge 34 and the second edge 36 of the material. The forming of the material can be accomplished either by utilizing the multi-screw extruder 24 and the die 26 discussed above or by manual processing without deviating from the subject invention. Further, it is to be appreciated that if the multi-screw extruder 24 of the manufacturing apparatus 20 discussed above is utilized then the material referenced in the following discussion would be the extrudate 30.

Generally, the extrudate 30 is heated to a temperature of from about 50 to 100 degrees Fahrenheit as it exits the multi-screw extruder 24. The method also includes heating the mandrel 38 to the first temperature. Typically, the first substance is applied to the first edge 34 and the second edge 36 and the material is wrapped around the mandrel 38. It is to be appreciated that the step of forming the first edge 34 and the second edge 36 of the material and applying the first substance to the first edge 34 and the second edge 36 occurs prior to the step of wrapping the material around the mandrel 38.

The wrapping of the material on the mandrel 38 occurs by abutting the first edge 34 and the second edge 36 of the material to define the spiral joint 40. The abutting of the edges 34, 36 is further defined as overlapping the planar first edge 34 with the planar second edge 36 in the preferred embodiment. The overlapping of the first edge 34 and the second edge 36 of the material occurs simultaneously with the step of abutting the first edge 34 and the second edge 36.

A pressure is applied to the spiral joint 40 for leveling the spiral joint 40 to form a smooth exterior surface 32 of the preform 22. The process of leveling the spiral joint 40 can consist of rolling the spiral joint 40 such that the exterior surface 32 of successive revolutions of the wrapped material are level with each other. Typically, the application of the pressure to level the spiral joint 40 ensures a strong bond between the first edge 34 and the second edge 36 when abutting each other. It is to be appreciated that the first edge 34 and the second edge 36 can overlap each other and application of the pressure forces the overlap edge into the material resulting in the exterior surface 32 of successive revolutions of the wrapped material being level with each other. The material defines the preform 22 once the exterior surface 32 of the material is leveled and the extrudate 30 is cut. The second substance can be applied to the exterior surface 32 of the material for assisting the pressure-applying device 42 to level the exterior surface 32. The pressure is applied to the spiral joint 40 after abutting the first edge 34 and the second edge 36 to form the spiral joint 40.

Typically, the method further includes the step of encapsulating the extrudate 30 or preform 22 with the film 48 while the extrudate 30 or preform 22 remains disposed on the mandrel 38 for controlling an evaporation rate of moisture from within the extrudate 30 or preform 22. It is to be appreciated that the encapsulation of the extrudate 30 or preform 22 can be accomplished by a variety of methods. The extrudate 30 or preform 22 is placed within the heating apparatus 52 and heated to a second temperature while the extrudate 30 or preform 22 is disposed on the mandrel 38 and encapsulated by the film 48. It is to be appreciated that the extrudate 30 or preform 22 may be heated to the second temperature by alternative methods without deviating from the subject invention.

The material or extrudate 30 has a temperature as the extrudate 30 exits the multi-screw extruder 24 of from about 50 to 100 degrees Fahrenheit. Preferably, the mandrel 38 is heated to the first temperature prior to wrapping the material or extrudate 30 on the mandrel 38 for preventing the material from cracking as a result of thermal expansion of the mandrel 38. Additionally, heating the mandrel 38 to the first temperature provides improved adhesion between the mandrel 38 and the extrudate 30 for improving the wrapping of the extrudate 30 on the mandrel 38.

Generally, the extrudate 30 or preform 22 is heated to the second temperature to effect drying of the extrudate 30 or preform 22. At this point, the extrudate 30 or preform 22 is known as an uncured ceramic article. Specific parameters and processes for heating uncured ceramic article are set forth in copending U.S. patent application Ser. No. ______, filed concurrently herewith (H&H Docket No. 65109.043).

The second temperature is from about 70 to 200 degrees Fahrenheit and more typically from about 110 to 130 degrees Fahrenheit. The extrudate 30 or preform 22 is held at the second temperature until a gel point of the organic binder is reached which occurs after about 90 to 240 minutes. It is to be appreciated that the extrudate 30 or preform 22 is maintained on the mandrel 38 during the steps of abutting, applying pressure and heating the extrudate 30 or preform 22 to the second temperature. The extrudate 30 or preform 22 is then removed from the heating apparatus 52 and the mandrel 38. It is to be appreciated that the several preforms 22 are disposed on the mandrel 38 up to this step and the removal of the preforms 22 requires removing the file 48 from the extrudate 30, cutting of the extrudate 30 to the desired width of the preform 22 and removing the preform 22 from the mandrel 38.

Once removed from the mandrel 38, the extrudate 30 defines the preform 22 and the preform 22 is placed into the heating apparatus 52 and again heated to the second temperature until the moisture content of the preform 22 is from about 0% to 18%, and more typically from about 5% to 10%.

The preform 22 is subsequently heated to a third temperature for about 30 to 90 minutes and more typically for about 60 minutes for burning off organic binders and fillers within the material. The third temperature is typically from about 450 to 700 degrees Fahrenheit and more typically from about 475 to 525 degrees Fahrenheit.

The preform 22 is then heated to a fourth temperature for about 90 to 150 minutes and more typically for about 105 to 135 minutes to set the inorganic binder and provide the ceramic article 10 with excellent strength at high temperatures. The fourth temperature is from about 1,600 to 2,000 degrees Fahrenheit and more typically from about 1,700 to 1,900 degrees Fahrenheit. After the step of heating the preform 22 to the fourth temperature, the preform 22 can be referred to as a cured or sintered ceramic article. The method of producing the preform 22 is completed by machining the preform 22 to a final configuration.

Obviously, many modifications and variations of the present invention are possible in light of the above teachings. The foregoing invention has been described in accordance with the relevant legal standards; thus, the description is exemplary rather than limiting in nature. Variations and modifications to the disclosed embodiment may become apparent to those skilled in the art and do come within the scope of the invention. Accordingly, the scope of legal protection afforded this invention may only be determined by studying the following claims. 

1. A method of forming a preform from a material having a first edge and a second edge utilizing a mandrel, the method comprising the steps of: forming the first edge and the second edge of the material with the first edge having a configuration complementary to a configuration of the second edge; wrapping the material around the mandrel; abutting the first edge and the second edge to form a spiral joint; and applying pressure to the spiral joint to create a smooth exterior surface of the material and to define the preform.
 2. The method of claim 1 further including the step of overlapping the first edge and the second edge of the material.
 3. The method of claim 2 wherein the step of overlapping the first edge and the second edge of the material occurs simultaneously with the step of abutting the first edge and the second edge to form a spiral joint.
 4. The method of claim 1 wherein the step of forming the first edge and the second edge of the material is performed before the step of wrapping the material around the mandrel.
 5. The method of claim 1 wherein the configuration of the edges is substantially planar and angled relative to an exterior surface and wherein the step of abutting is further defined as overlapping the planar first edge with the planar second edge.
 6. The method of claim 1 further including the step of applying a substance to the first edge and the second edge for increasing adhesion of the material prior to the step of wrapping the material around the mandrel.
 7. The method of claim 1 further including the step of applying a second substance to the spiral joint prior to applying pressure to the spiral joint.
 8. The method of claim 1 wherein the step of applying pressure to the spiral joint is performed after the step of abutting the first edge and the second edge to form a spiral joint.
 9. The method of claim 1 further including the step of heating the mandrel to an elevated temperature before the step of wrapping the material around the mandrel.
 10. The method of claim 1 wherein the step of abutting the first edge and the second edge to form a spiral joint is performed simultaneously with the step of wrapping the material around the mandrel.
 11. The method of claim 1 further including the steps of maintaining the preform on the mandrel during the steps of abutting and applying pressure and subsequently removing the preform from the mandrel.
 12. The method of claim 1 further including the step of encapsulating the preform with a film while the preform remains disposed on the mandrel for controlling an evaporation rate of moisture within the preform.
 13. The method of claim 12 further including the step of heating the preform to an elevated temperature while disposed on the mandrel.
 14. The method of claim 13 further including the steps of removing the film from the preform and drying the preform.
 15. The method of claim 1 further including the steps of heating the mandrel to a first temperature, heating the preform to a second temperature which is equal to or lower than the first temperature while on the mandrel, and heating the preform to a third temperature greater than the first and second temperatures for burning off organic binders and fillers within the material and heating the preform to a fourth temperature greater than the first, second and third temperatures for setting an inorganic binder and providing the preform with excellent strength at high temperatures.
 16. The method of claim 1 further including the step of machining the preform to a final configuration.
 17. A method of forming a ceramic preform having ceramic particles and ceramic fibers with an aspect ratio of greater than 3:1 and the ceramic fibers substantially randomly orientated in three dimensions utilizing a mandrel and an extruder, the method comprising the steps of: extruding the ceramic particles and ceramic fibers through the extruder to form an extrudate having a first edge and a second edge; forming the first edge and the second edge of the extrudate with the first edge having a configuration complementary to a configuration of the second edge; wrapping the extrudate around the mandrel; abutting the first edge and the second edge to form a spiral joint; and applying pressure to the spiral joint to create a smooth exterior surface of the extrudate and to define the preform.
 18. The method of claim 17 wherein the step of forming the first edge and the second edge of the extrudate is performed simultaneously with the step of extruding the ceramic particles and ceramic fibers through the extruder.
 19. The method of claim 17 further including the step of overlapping the first edge and the second edge of the extrudate.
 20. The method of claim 19 wherein the step of overlapping the first edge and the second edge of the extrudate occurs simultaneously with the step of abutting the first edge and the second edge to form a spiral joint.
 21. The method of claim 17 wherein the step of forming the first edge and the second edge of the extrudate is performed before the step of wrapping the extrudate around the mandrel.
 22. The method of claim 17 wherein the configuration of the edges is substantially planar and angled relative to an exterior surface and wherein the step of abutting is further defined as overlapping the planar first edge with the planar second edge.
 23. The method of claim 17 further including the step of applying a substance to the first edge and the second edge for increasing adhesion of the extrudate prior to the step of wrapping the extrudate around the mandrel.
 24. The method of claim 17 further including the step of applying a second substance to the spiral joint prior to applying pressure to the spiral joint.
 25. The method of claim 17 further including the step of heating the mandrel to an elevated temperature before the step of wrapping the extrudate around the mandrel.
 26. The method of claim 17 wherein the step of abutting the first edge and the second edge to form a spiral joint is performed simultaneously with the step of wrapping the extrudate around the mandrel.
 27. The method of claim 17 further including the steps of maintaining the preform on the mandrel during the step of abutting and subsequently removing the preform from the mandrel.
 28. The method of claim 17 further including the step of encapsulating the preform with a film while the preform remains disposed on the mandrel for controlling an evaporation rate of moisture within the preform.
 29. The method of claim 28 further including the step of heating the preform to an elevated temperature while disposed on the mandrel.
 30. The method of claim 29 further including the steps of removing the film from the preform and drying the perform.
 31. The method of claim 30 further including the steps of heating the mandrel to a first temperature, heating the preform to a second temperature which is equal to or lower than the first temperature while on the mandrel, baking the preform at a third temperature which is greater than the first and second temperatures and subsequently firing the preform to a fourth temperature with is greater than the first, second and third temperatures for increasing a strength of the preform.
 32. A manufacturing apparatus for forming a ceramic preform from an extrudate having a first edge and a second edge, said apparatus comprising: a multi-screw extruder having at least three intermeshing screws for forming the extrudate; a mandrel having a diameter for receiving the extrudate with the extrudate wrapping around said mandrel to form a spiral joint by overlapping and abutting the first edge with the second edge; a pressure-applying device adjacent to said mandrel for engaging the extrudate and applying pressure to the spiral joint to create a smooth exterior surface of the extrudate and to define a preform.
 33. An apparatus as set forth in claim 32 wherein the multi-screw extruder has at least three intermeshing screws rotating at 20 to 1,200 rpm.
 34. An apparatus as set forth in claim 32 wherein the multi-screw extruder includes at least three screws formed in a ring configuration.
 35. An apparatus as set forth in claim 32 wherein the multi-screw extruder has 2 to 8 mixing zones.
 36. An apparatus as set forth in claim 32 wherein the multi-screw extruder has an L/D ratio of about 18 to
 56. 37. The apparatus as set for in claim 32 wherein the mandrel is heated to an elevated temperature.
 38. The apparatus as set forth in claim 32 further including a wrapping device for encapsulating the preform with a film to control the evaporation rate of moisture within the preform.
 39. The apparatus as set forth in claim 32 further including a heating apparatus for heating the preform to an elevated temperature and for baking and firing the preform after the film has been removed for increasing a strength of the preform.
 40. The apparatus as set forth in claim 32 further including a die mounted to said extruder for shaping the extrudate. 